epubcheck error message

Hello,
I got the following error message from the epubcheck validator. Can anyone explain what it means so that I can rectify the problem? I tried making a few changes to my InDesign print document file but to no avail. Please bear in mind that I am not au fait with html.
Many thanks,

Well, look into the mentioned file around the mentioned line and position.
Presumably there is an empty element a or with some none text content like an image, because this is restricted by EPUB for the navigation document.

Thanks so much for your suggestion O.H. but I don't know how to identify line 34 and position 90. Sorry to ask what, to you, may sound obvious, but does "line 34" mean the 34th line number on that particular page or from the beginning of the document? And what does "position" refer to?
I'd truly appreciate any help on this. I feel I've come a long way on my learning curve to export epubs from InDesign but I am still a novice methinks.

hey..i've got the same issue but i double checked the whole document and it seems like no anchor text is missing..how do i fix it??and better asap..since need to upload the project to http://www.domyhomework4me.net/ ..can this be some kind of a bug??should i reinstall the program??